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The Interaction Between Dislocations and Lamellar Grain Boundaries in Pst γ Tiai

Published online by Cambridge University Press:  21 February 2011

S. Rao ,
C. Woodward  and
P.M. Hazzledine
S. Rao
Affiliation:
UES Inc., 4401 Dayton-Xenia Road, Dayton, OH 45432
C. Woodward
Affiliation:
UES Inc., 4401 Dayton-Xenia Road, Dayton, OH 45432
P.M. Hazzledine
Affiliation:
UES Inc., 4401 Dayton-Xenia Road, Dayton, OH 45432
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Abstract

In lamellar TiAl the flat-plate geometry of the grains, the barriers to deformation across the grain boundaries and the coherency stresses all contribute to a marked anisotropy in the yield and fracture stresses of the material. Both yield and fracture occur at low stresses when the deformation is within the lamellae (soft mode) and they occur at high stresses when the deformation crosses the lamellae (hard mode). The anisotropy is enhanced by a new effect which can soften the soft mode and harden the hard mode: the geometry of the lamellar boundary produces degeneracies in the planar fault energies at the interfaces which enhance the mobilities of dislocations on these interfaces. These degeneracies modify the core structure of dislocations on or near the interfaces, consequently soft mode dislocations can dissociate widely and move more easily when their glide plane is contained in the interface. Hard mode dislocations can substantially reduce their core energies when intersecting a γ/γ interface, and subsequently become immobilized, by cross slipping on to the interface plane. This paper presents a discussion of the geometry and relative energies of the γ/γ interfaces using elements of Bollman O-lattice theory. In order to investigate the influence of the interfaces on dislocation core structure we have fit an empirical Embedded Atom Method (EAM) potential to the structural and elastic properties of bulk L10 TiAl. The mobility and core structure of the twinning dislocation at the 180° interface and the perfect, 1/2<110] screw dislocation at the 60° and 120° interfaces were calculated using molecular statics within the EAM. We have also studied the influence of one and two atomic step ledges on dislocation mobility in the 120° interface. We find in general that dislocations are more glissile on the γ/γ interfaces, as compared to bulk TiAl and that ledges are weak barriers to dislocation glide. The interfaces themselves are strong barriers to dislocation motion in the hard mode. We find that the 1/2<110] screw dislocations gliding on conjugate {111} planes are trapped at these interfaces, as a result of lower core energies for screw dislocations lying in the interface.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 319: Symposia CB – Defect-Interface Interactions , 1993 , 285
Copyright
Copyright © Materials Research Society 1994

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References

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